In the refining of nonferrous metals, it is common to remove impurities from the molten metal by a so-called "fire-refining process" whereby the metal is first oxidized by treatment with air, oxygen-enriched air or oxygen, thereby volatilizing or slagging the impurities and forming small amounts of metal oxide which is dissolved in the charge of molten metal with which the process starts. The metal oxide is reduced by poling or other treatment of the molten metal bath with a pulverulent or liquid reducing agent or by reducing gases.
The two processes can be carried out successively in a single unit, thereby resulting in batch production of the refined mteal, or can be carried out intermittently or continuously in two separate units, one of which serves for the oxidation of the impurities while the other carries out the reducing step.
In the refining of copper (see German Pat. No. 810,432) it is known to use a rotatable drum furnace which is charged with the liquid copper and is subdivided internally by a partition into an oxidizing chamber and a reducing chamber, means being provided to remove the slag while a siphon is formed in the partition or by the partitions to separate the oxidizing and reducing chamber.
More specifically, the slag is discharged through a slag hole in the shell or wall of the drum forming the furnace while the refined copper is discharged into a separate collecting vessel, on wheels, through a discharge spout provided in an end wall of the furnace.
This system has the disadvantage that it cannot be operated continuously since, when the collecting vessel is filled, the collecting unit must be separated from the remainder of the installation and wheeled into the foundry for discharge.
Consequently, the delivery of copper from this unit and the charging of metallic copper into the unit is interrrupted for the period this requires.
Another disadvantage of the earlier system is that it generally must be constructed in small units to facilitate mobility and hence the semibatch process which results does not afford a constant quality of the copper produced over a long operating period.